1. Technical Field
The present disclosure relates to a behavior control apparatus for a vehicle such as an automobile.
2. Description of the Related Art
A behavior control apparatus that performs an understeer control for reducing a degree of understeer state by controlling braking/driving forces of wheels in a vehicle such as an automobile when the vehicle is in an understeer state is known. In a behavior control apparatus performing an understeer control, for example, as described in Japanese Patent Application Laid-open No. H07-69230, it is determined whether or not a vehicle is in an understeer state by determining whether or not a magnitude of a deviation between a target yaw rate of a vehicle based on a steering angle and a vehicle speed and a detected actual yaw rate of the vehicle is equal to or larger than a threshold value. It is also known that an understeer state of a vehicle is determined based on a deviation between a steering angle and a value (a steering angle conversion value of a yaw rate) obtained by converting a detected actual yaw rate of the vehicle into a steering angle.
As is well known, in a steering apparatus for a vehicle such as an automobile, rotation of a steering wheel operated by a driver is transmitted to a rack and pinion device via a steering shaft, and is converted into linear motion of the rack bar in the vehicle lateral direction by the rack and pinion device. The linear motion of the rack bar is transmitted to the wheels via tie rods and knuckle arms, whereby the wheels are steered. The rack and pinion device includes a rack bar that meshes with a pinion coupled to the steering shaft.
The ratio of a steered angle of wheels to a rotation angle of the steering wheel, that is, a steering transmission ratio (a reverse value of a steering gear ratio) is preferably larger in a range where a magnitude of a steering angle is large than in a range where a magnitude of a steering angle is small. Therefore, a so-called variable gear ratio type rack bar may be used in which a pitch of the rack teeth in a region in the vicinity of the both ends corresponding to a range where a magnitude of a steering angle is large is larger than a pitch of the rack teeth in the other region corresponding to a range where a magnitude of a steering angle is small. According to the variable gear ratio type rack bar, it is possible to increase the steering transmission ratio in a range where a magnitude of the steering angle is large as compared with a general rack bar in which a pitch of the rack teeth is constant over the entire region.
In a vehicle in which a rack bar is a variable gear ratio type rack bar, when an understeer state of a vehicle is determined using a certain threshold value based on a deviation between a steering angle and a steering angle conversion value of a yaw rate, erroneous determination tends to occur in a range where a magnitude of a steering angle is large, that is, in a range where a steering transmission ratio is large.
In a yaw rate sensor for detecting a yaw rate of a vehicle, a so-called zero point offset may occur where a detected yaw rate becomes a value other than 0 even when the vehicle is in a straight traveling state. When a zero point offset occurs in a yaw rate sensor, a steering angle conversion value of a yaw rate becomes a value different from a value obtained by converting an actual yaw rate of the vehicle to a steering angle. Therefore, as will be described in detail later, it is determined that the vehicle is in an understeer state although the vehicle is not in an understeer state, the braking/driving forces of the wheels are controlled unnecessarily, and a driver may feel annoyance. Conversely, it is determined that the vehicle is not in an understeer state despite the vehicle being in an understeer state, and required control of braking/driving forces of the wheels may not be executed.
Note that a situation where it is determined that a vehicle is not in an understeer state despite the vehicle being in an understeer state and the necessary braking/driving force control of the wheels is not executed is the same as the situation in a vehicle in which the understeer control is not performed. Since a driver tries to deal with this situation by increasing steering, he or she does not feel annoyance due to this situation.